[core] add gfx::CommandEncoder::present()

include/mbgl/gl/renderable_resource.hpp

@@ -2,6 +2,8 @@
 
 #include <mbgl/gfx/renderable.hpp>
 
+#include <cstdlib>
+
 namespace mbgl {
 namespace gl {
 
@@ -11,6 +13,11 @@ protected:
 
 public:
     virtual void bind() = 0;
+
+    virtual void swap() {
+        // Renderable resources that require a swap function to be called explicitly
+        // can override this method.
+    }
 };
 
 } // namespace gl